Wood treatment method

ABSTRACT

AFTER WOOD HAS BEEN IMPREGNATED WITH AN AQUEOUS SOLUTION OF A WOOD PRESERVATIVE IT IS DRIED BY HEATING IT IN A HEATED BATH OF HIGH-BOILING OIL IN A CHAMBER, UNDER VACUUM, THE TEMPERATURE AND SUBATMOSPHERIC PRESSURE BEING ADJUSTED TO REMOVE THE SOLVENT AT A TEMPERATURE BELOW THE NORMAL BOILING POINT OF THE SOLVENT.

United States Patent 3,560,251 WOOD TREATMENT METHOD Bror Olof Hager,Djursholm, Sweden, assignor to Hager Aktiebolag, Stockholm, Sweden NoDrawing. Filed July 11, 1966, Ser. No. 563,979 Claims priority,application Germany, July 19, 1965, P 14 92 511.2 Int. Cl. Bc 11/10;B27k 3/02 US. Cl. 117102 5 Claims ABSTRACT OF THE DISCLOSURE After Woodhas been impregnated with an aqueous solution of a wood preservative itis dried by heating it in a heated bath of high-boiling oil in achamber, under vacuum, the temperature and subatmospheric pressure beingadjusted to remove the solvent at a temperature below the normal boilingpoint of the solvent.

The present invention relates to a method of treating wood (and similarorganic substances) to combat decay and damage by insects. According tothe complete method, the so-treated material is dried and substances maybe, or are, added thereto which give it different valuable properties.The method is carried out in the following way.

The wood is treated with a preserving solution, preferably underpressure, according to usual standard processes. In order to facilitatethe subsequent drying it is desirable to introduce no more solvents thannecessary into the wood. Preferably the Lowry or Ruping treatment is tobe observed (as opposed to full cell or open tank treatment). Thepreserving treatment is followed by drying the treated woodpreferably,under vacuun1in a warm high-boiling liquid which is immiscible with thepreserving solution.

The preference for the Lowry or Ruping procedure stems from thefollowing facts.

By the full cell treatment, the wood is exposed to a vacuum before thesolution is introduced under pressure into the wood. This treatmentgives the highest possible retention of solution in the wood, in thatnot only the walls of the cells in the wood are saturated with solutionbut also the cells are filled with solution. As an example, it can benoted that in Swedish pine treated by full cell treatment there isusually introduced about 300 liters of solution per cubic meter of wood.The Lowry treatment, on the other hand, starts without initial vacuum,the solution being directly introduced into the wood by pressure. Thenthe solution in the pressure cylinder (or, equivalent pressure vessel)which has not been actually introduced into the wood is removed from thecylinder by a pump, and a vacuum is applied to the interior of thevessel. Part of the solution introduced into the wood is recovered fromthe wood by means of the compressed air introduced into the wood duringthe pressure period. By treating Swedish pine according to the Lowryprocess usually about 150 liters of the preserving solution areintroduced into, and well distributed in, the wood.

The Ruping treatment is still further developed in order to effect a lownet retention. It is begun with an air pressure of some kilograms on thewood before the solution is introduced. This means that an increased airquantity is compressed within the wood when the solution thereafter isintroduced under an elevated pressure. In the subsequent vacuum periodmore solution is recovered than in the Lowry treatment. When treatingSwedish pine according to the Ruping treatment usually at the most 100liters of solution per cubic meter are introduced into the wood. In thisrespect, the Ruping treatment has some technical advantages. Theapparatus for its use is, how- "Ice ever, so expensive that in spite ofthe higher amount of solution introduced, the Lowry treatment generallyis to be preferred for the process according to the present invention.

By the different impregnating treatments different quantities ofsolution are thus introduced into the wood. Therefore also differentconcentrations of the solutions are used, corresponding to the amount ofsolution introduced so that the wood always obtains the correct (e.g.,the same) amount of preservative per unit volume.

According to the invention, it is the intention in the first line to useaqueous solutions of preservatives which, after the treatment, are fixedin the wood. In some instances, the preserving substance is dissolved inwater by means of ammonia and ammoniacal salts. Solutions of this kindare, for example, described in US. Pats. 2,908,- 607 and 3,089,810. Theyhave the important property of being stable enough to be used in theLowry and Ruping treatments.

After the impregnating treatment has been carried out and thepreservative solution has been introduced and distributed in the woodand excess solution has been pumped away from the pressure cylinder a,high boiling liquid is introduced. By high boiling liquid is here meanta liquidfor instance, an oilboiling over 100 C., preferably, over 250300C. The wood is dried by the use of this oil. This is done by keeping theoil at an elevated temperature, and keeping the cylinder (with the woodin it) under a high vacuum. In order to obtain a mild drying, and toavoid inconveniences, e.g., foaming, from arising, the different momentsof the drying process can be increased successively to full effect.These moments are the introduction of the oil into the cylinder, itsheating and the raising of the vacuum. Antifoaming means can also beused. The final temperature of the oil ought to be about C., but higherand lower final temperatures can be tolerated depending upon theresistance of the wood. The normal variation in the final temperaturecan be said to be between 60 and C. In this connection it is noted thathigh temperatures cause risks of damage in the wood in the form ofwarps, cracks, tensions within the wood, etc. Such risks are normallynot present when drying at 80 C. When treating easily damaged woods, thefinal temperature has to be somewhat decreased, with some consequentprolongation of the driving time.

Steam coming from the wood gives the oil an effective stirring and,therefore, a good and even heat transfer is obtained. Accordingly,special arrangements in the cylinder for circulating the oil are notneeded. When the drying begins, the wood contains much preservingsolution and is, therefore, relatively very heavy, and hence it sinks inthe light medium which consists of the oil highly mixed with steambubbles. Gradually, the wood dries and grows lighter: at the same timethe oil becomes less and less mixed with steam bubbles and hence themixture of oil and steam bubbles grows heavier. The wood eventuallybegins to float. During a prolonged drying procedure the wood and theoil-steam mixture thus have about the same weight per volume and thisgreatly favors the contact between oil and wood, and likewise favors theeffective execution of the drying process. The pressure between thedifferent wood pieces will therefore not be so heavy, and layingcrosspieces between the different wood pieces is, as a rule, notnecessary. All these circumstances promote the rate of heating of theWood. By the additional vacuum, the drying proceeds both ex tremely fastand mild, since the water evaporates from the wood by boiling underreduced pressure.

A fast drying above the boiling point, without the aid of vacuum, is outof question, because the high temperatures which are needed in such acase tend to cause damages in the wood.

After the wood has been dried, the oil is pumped out of the cylinder.This has to be done with maintained vacuum, unless an exceptionally highamount of oil in the wood is wanted. If the vacuum is broken with theoil left in the cylinder the consequence will be that the air pressurewill force oil into the wood, the cells of which in this moment will beevacuated. They have been filled with steam at a low pressure, and whenthe vacuum is broken the steam condenses. After the oil has been removedfrom the cylinder the wood is taken out. The treatment is thencompleted. The so-treated wood has protection against wood destroyers:it has been dried andowing to the oil in the outer layer of the wood-hasacquired water repellency which means that the wood swells and shrinksless than does wood which has not undergone this treatment.

The oil in which the wood is dried can be parafiin oil or other highboiling oil; however, one had better use different drying oils, such aslinseed oil or linseed oil mixtures. Several oils of drying character inthe petrol industry are suitable either as they occur or in differentmixtures. If paraffin oil or other nondrying oil is used the surface ofthe wood will often be a little smeary, which condition will not happenwhen drying oils are being used. Of course it takes some days before theoils dry so that all smear disappears.

Different substances can be introduced into the oil in which the wood isheated, in order to give the wood different wanted properties; or, theoil can be replaced by some substances of this kind. The drying can thusbe carried out in solutions or in melts of waxes and/ or resins in orderto obtain an increased water-repellency or a hard surface. As furtherexample chloroparaflin can be mentioned: this gives the wood certainfire-protecting properties which is or may be desirable. Chloroparaffincan be used alone or dissolved in a high-boiling oil. If the proportionof chlorine in the chloroparafiin is high it is difficult to use itwithout mixing it with oil. Other organic substances containing chlorineor phosphorus have fireprotecting properties and can be used in asimilar way. An illustrative commercial product of this kind containingphosphorus is called Phosgard. Good results have been obtained withtricresylphosphate and similar substances. They can with advantage beused together with drying oils. For example, tricresylphosphate andlinseed oil in equal parts is a suitable composition. The so-obtainedfire protection has been very apparent and it has also had the veryimportant property to be resistant against leaching.

Furthermore, suitable coloring substances can be introduced into the oilin order to color the wood in the course of the drying. Different kindsof color substances have been investigated: best permanence has beenobtained with pigment colors, which have proven to be superior to colorsdissolved in the oil. The pigments have to be very finely subdivided;moveover, they should emulsify readily. Otherwise, they are not easy touse.

To promote ease in carrying out the process in practice, some detailsought to be observed. Thus, it is important to remove all solution fromthe cylinder after the treatment of the wood. No pockets where solutioncan remain should occur in the tapping system. In such a case theretained solution will be evaporated to dryness during the followingdrying of the wood in the oil. The preserving agents present in theresidual solution form solid products under such circumstances, whichsolid products have to be removed separately or they will form coveringson wood and on apparatus details. In order to ensure against this risk,one can introduce into the oils (in which the wood is dried) somewater-insoluble substances which are solvents for the preserving agentsand solid products formed thereof, Products with such dissolvingproperties are, for instance, tall oil and naphthenic acid.

Another practical detail is to remove the oil from the cylinder afterthe drying period so that no residual oil will be left. Such residuewould be mixed with the treating solution in continuous operation. Thewarm oil is comparatively easy to remove as it flows readily in thisstate. An especially effective way to remove oil residues is to steamthe cylinder with the wood after the removal of the oil. By such atreatment both apparatus and wood will be cleaned very fast from oilresidues on their surfaces.

As above stated, the process is chiefly adapted for drying wood that hasbeen treated with aqueous treating solutions. On the other hand, it isoperable in cases in which the preservative is dissolved in an organicsolvent. Wood treated in this way can be dried in a high boiling liquidin which the organic solvent is insoluble. The solvent which evaporatesduring the drying period can be condensed and recovered. Such organicsolvents insoluble in paraffin oil are, for instance, alcohols andorganic acids.

EXAMPLE As an example of how the process can be carried out thefollowing may be given:

Pine wood of one and two inches thickness was treated with an aqueousammoniacal solution containing 0.6 percent copper, 0.3 percentchlorophenol, 1.2 percent am monia and 2.5 percent carbon dioxide,percent here meaning percent by weight. The treating solution was forcedinto the wood under 8 kilograms per square centimeter pressure during 90minutes, whereafter the solution was removed from the impregnatingcylinder and a high vacuum was applied during 30 minutes. A considerableamount of solution was evacuated from the wood and was removed from thecylinder by a pump. With maintained high vacuum, a mixture of oilshaving a temperature of 40 C. was successively introduced into thecylinder. The oil mixture contained equal parts of linseed oil and aslowly drying, high-boiling oil and in the oil mixture a brown colorpigment was dispersed. The pigmented oil mixture was heated by steam inpipelines in the bottom of the treating cylinder in a scant hour up toC.

At this final temperature, and under a high vacuum, the drying of thewood proceeded during 4 hours. The oil thereafter was removed from thecylinder in which the vacuum was maintained until all free oil was eXpelled. Steaming was then carried out during 10 minutes in order toremove oil residues. The wood was then taken out from the cylinder.

The product had been treated in respect to conservatron, to drying, towater-repelling properties and to coloring. The waterwhich in the formof solution had been introduced into the wood during the treatment-hadbeen removed for the most part. The amount of oil absorbed by the Woodwas less than 40 kilograms per cubic meter, and was to be found in theoutermost layers of the wood.

An interesting variation of the treating method of the present inventionis the treatment of moist (green) wood. The introduction of the solutioninto the wood can hereby be carried out in the form of a diffusiontreatment and this can be used for the treatment of practically allkinds of wood, including varieties which in the dry state are difficultto treat under pressure. Also, in these cases it is an advantage to useammonia and/or ammoniacal salts as dissolving agents. In this waypreservatives are obtained which can both penetrate into the moist woodby diffusion and also be fixed in the Wood after drying. A suitablesolution for treatment of green wood according to the diffusion methodis an aqueous solution containing 1.2 percent copper, 0.6 percentarsenic pentoxide, 4 percent ammonia and 5 percent carbon dioxide. Thecarbon dioxide stabilizes the solution and makes it more resistant toevaporation and hydrolysis. It can be replaced by other acids such asacetic or formic acid.

In carrying out the process just mentioned, the moist wood is kept inthe preservative solution a certain time during which the preservativepenerates by diffusion into the wood. The time for this treatment varieswith the dimensions and the kind of wood. As a general rule it may besaid that one-inch-thick wood usually is kept in the solution a weekwhile poles are treated for a month. After a sutficient amount ofpreservative has penerated into the wood the preservative solution isremoved. Preservative has now entered into the outer layers of the wood.It has to be distributed further into the wood: accordingly, the woodshould not be dried but rather be kept in moist state for some time forpurposes of equalization of distribution. This distribution time shouldbe considerable: it ought to be at least as long-and may amount to asmuch as three times as long-as was the time interval during which thewood has been kept in the preserving solution. The drying may be avoidedby covering the wood, keeping it enclosed or surrounded by oil duringthe distribution period. If the oil is kept warm, i.e., at 4050 C., thedistribution of the preservative proceeds faster and this period can beshortened. Thereafter, the above-described drying in oil follows andthen the treatment of the wood is finished.

When treating moist wood, pressure can be used with advantage-a fullcell treatment is in this case to be preferred-or an open tank treatmentcan be used instead of keeping the wood a long time in the solution toobtain the penetrating diffusion. Such treatments are specially usefulwhen the wood has been dried to some extent and thus is not quite moistor green. By these treatments more preservative (solution) is usuallyintroduced into the wood than by diffusion treatments. Therefore it isdesirable to use a weaker solution-for instance, a solution containinghalf the amount of the ingredients used for the diffusion solution, thesaid half amount being the same as is used for the Lowry treatment. Withsuch a solution moist pine wood can be treated with remarkably goodresults. Of important interest is the fact that even spruce and otherkinds of wood which are difiicult to impregnate by ordinary methods canbe successfully treated.

As long as the wood is moist it is permeable for the impregnatingsolution. A high vacuum is applied to the wood in the treating cylinder,whereafter the solution is introduced and is forced into the wood undera pressure of, say, 8 kilograms per square centimeter. The free solutionis then removed from the cylinder, and a suitable oil is introduced,which oil at 80 C. dries the wood under a high vacuum as earlierdescribed. It is here to be observed htat a longer drying time isnecessary because the wood contains much more water than after a Lowrytreatment.

By treating moist wood it is desirable to use methods which give thewood the highest possible amount of preservative solution. Depending onthe water content the air space in the wood cells is definitely smallerthan in dry wood, and for this reason also the possibility ofintroducing preserving solution is much less. As a rule, it is mostsuitable to fill this cell space completely with solution. In moist woodthe air space in the cells is too small to be used as intended whencarrying out the Lowry or Ruping treatment. When treating moist wood,the diflFerent pressure methods therefore give similar results. By fullcell treatment of moist wood, however, often so good a penetration anddistribution can be obtained that the drying in oil can start at oncethereafter: a distribution period is thus not necessary. When thedemands on the treatment are not so high this way can be used withadvantage. It is to be observed that during the drying period anapparent distribution of the preservatives occurs. Here we thus have thevery favorable situation that undried wood can be preserved, dried,colored, etc., and ready for delivery in one day. The only question leftafter the treatment is that the drying oils, if used, need some days forbeing solidified.

As said above, it is a considerable advantage to use the Lowry treatmentfor handling the drying of the wood in the best way. Depending on thedifferences in apparatus and in the conditions of the wood it isdifficult to give exact figures for the drying times. As a rule, 4 to 6hours drying time is used for sawn Wood when the Lowry process isapplied. During this time usually more than percent of the waterintroduced into the wood will have been evaporated. For largerdimensions and poles, the drying times often are considerably longer. Inorder to obtain effective operation, the process can thereby befull-filled during a night shift. Drying times of for example 16 to 20hours can then be used without disadvantage and without noticeablecapacity decrease.

When full cell treatment of moist wood is used, the drying process isprolonged and it is also more difiicult to obtain even drying. It ishere impossible to give general rules. Very often, however, the dryingtime will be two or three times longer by full cell treatment than iscalled for by Lowry treatment.

I claim:

1. Method of preserving wood which comprises treating the same, in aclosed space and at a pressure at least as high as atmospheric pressure,with an aqueous solution of a wood preservative, which after thetreatment is fixed in the wood in nonleachable condition, treating theso-impregnated wood, in said enclosed space, in a heated liquid bath ofa high-boiling medium, with said space being maintained undersubatmospheric pressure and the temperature of the bath and thesub-atmospheric pressure being adjusted to remove the water present inthe impregnated wood at a temperature below the normal boiling point ofwater and within the range of from about 60 to about C. and withdrawingsaid high-boiling medium from the so-treated wood in said space whilstsaid space is maintained at subatmospheric pressure, whereby to minimizeretention of said medium by the so-treated wood.

2. The improved method defined in claim 1, in which the high-boilingmedium is selected from the group consisting of high-boiling nondryingoils, drying oils and mixtures thereof, and in which the high-boilingmedium boils over 250-300 C.

3. Method according to claim 1, in which the aqueous preserving solutioncontains copper compounds dissolved in ammonia compounds givingdifficulty soluble preserving copper compounds in the wood after thetreatment and the evaporation of the ammonia compounds.

4. Method according to claim 1, in which fire retardants are dissolvedin the high boiling liquid.

5. Method according to claim 1, in which fine granular colour pigmentsare dispersed in the high boiling liquid.

References Cited UNITED STATES PATENTS 247,602 9/1881 Boulton 117-116360,947 4/1887 Boulton 117-116 109,874 12/1870 Cresson 117-102 990,2464/ 1911 Fetterman 34-95 1,756,797 4/1930 Rawson 117-102 2,892,261 6/1959Hutchinson 34-95 3,061,508 10/1962 Morriss et al 21-7X 3,200,003 8/ 1965Bescher 21-65X FOREIGN PATENTS 159,784 11/1954 Australia 117-119.8

OTHER REFERENCES Special Methods of Seasoning Wood-Boiling in Oil,Forest Products Laboratory Report No. R1665, Madison, Wis., 1947.

Hunt, George M., and Garratt, George A.-: Wood Preservation,McGraw-Hill, New York, 1938 (pp. 127- 129 and 409-410 relied on).

MORRIS O. WOLK, Primary Examiner B. S. RICHMAN, Assistant Examiner US.Cl. X.R.

